In the manufacture of thermoplastic foams, the thermoplastic resin is rendered molten or fluid by the application of heat and/or mechanical working, and the molten plastic resin is then admixed under pressure with a volatile blowing agent, such as a fluorocarbon, to form a solution. This solution, of appropriate viscosity, is passed through a constricted orifice or die into a region of lower pressure, usually atmospheric pressure, causing the dissolved blowing agent to volatilize and form a mass of fine gas bubbles within the body of the fluid resin. Through a variety of processes, the chief of which is the loss of heat, the intimate gas/liquid mixture becomes a gas/solid foam. Subsequent processes such as thermoforming, molding, and trimming may be used to convert the foam into desirable, useful shapes and sizes.
The recovery and reuse of the volatile blowing agent used in foam production would be of obvious economic benefit to the manufacturer, since the blowing agent is a major cost factor in the production of the foam. For example, the production of polyethylene foam may typically use 30 to 40 lbs. of blowing agent for each 100 lbs. of polyethylene resin used. However, until the present invention, no process or apparatus has been developed which will permit recovery and reuse of the blowing agents used in foam production. Thus, the conventional practice in commercial foam production processes has simply been to permit the blowing agents to escape into the atmosphere.
In recent years, there has been a great deal of concern over the harmful or potentially harmful effects of some volatile organic substances in the earth's atmosphere. In particular, there have been a number of studies directed to the effects of fluorocarbon emissions into the atmosphere, and efforts have been made to restrict fluorocarbon emissions in various industrial processes and in consumer items such as aerosol sprays. The present invention now makes it technically possible and economically feasible to collect and recover fluorocarbon blowing agents used in foam production so as to eliminate or significantly reduce the discharge of such agents to the atmosphere.
In order to recover the blowing agent in a form suitable for reuse, the agent must be converted into the form that it will be used in the foam production process, i.e. a liquid, and the blowing agent must be free from contaminants, most notably air. To accomplish this separation from air, the main contaminant, is a primary objective of the present invention.
Processes for recovering volatile compounds such as those used as blowing agents in foam production have been previously used in a number of industrial processes other than foam production. Generally, however, recovery of these materials has only been feasible in processes where it is possible to collect and recove the volatile material at a relatively high concentration without the presence of a large amount of other contaminants, such as air. Examples of several known recovery systems are disclosed in the below-listed U.S. patents:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,780,744 Neel et al December 25, 1973 3,788,331 Neel et al January 29, 1974 3,793,801 Tsao February 26, 1974 4,095,605 Conrad June 20, 1978 4,175,932 Durr et al November 27, 1979 4,289,505 Hardison et al September 15, 1981 ______________________________________
The present invention is based upon the recognition that substantial differences exist in the location where the blowing agent is lost from the foam and the amount of blowing agent which is lost, depending upon the particular process and especially upon the exact nature of the resins employed, as well as upon the blowing agent. Resins with substantial aliphatic content, as for example the polyolefins, lose a very substantial portion of the blowing agent in the vicinity of the extrusion die and offer an excellent opportunity for collection and recovery at that location. Polymers with substantial aromatic content such as the styrenic polymers, or heteroatoms, such as the vinyl acetate and vinyl chloride polymers tend to retain the blowing agent for a longer time, and offer additional opportunities for recovery during subsequent processing such as in regrinding of scrap or crushing of the foam. Still, however, a very substantial proportion of the blowing agent is lost in the vicinity of the extrusion die.